Diaphragm pumps use an offset eccentric bearing, a piston to help create the motion that moves the diaphragm and causes pressurized paint flow. As the motor turns, the eccentric bearing turns. One end of the piston rides on this bearing. The turning of the eccentric bearing creates up-and-down movement of the piston. As the piston goes down, hydraulic oil is drawn into the cylinder from the hydraulic oil reservoir. While the piston is going down, the diaphragm spring pushes the diaphragm down. As the diaphragm is pushed down, the inlet valve is sucked open. A "gulp" of paint is then sucked though the inlet valve and into the paint pump.